The ground track oblique Cassini projection used for producing VIIRS mapped imagery

Abstract

The Suomi-NPP Visible Infrared Imager Radiometer Suite (VIIRS) radiance is mapped to make image products using the ground-track Mercator (GTM) algorithm developed at Raytheon. This algorithm defines a process for transforming gridded map (x/y) coordinates of the image into Earth coordinates (longitude/latitude). The y-axis reference is the satellite ground track, which is mapped with an even scale. Great circles placed orthogonally with respect to the ground track define the x-axis. In its current state the algorithm is only defined for the Map-to-Earth (MtE) transformation, but the Earth-to-Map (EtM) transformation has no mathematical algorithm, and instead must use a slow search algorithm for every point. Because of this, the GTM is not a true map projection. This paper remedies this by describing an EtM transformation algorithm using a Ground Track Oblique Cassini (GTOC) projection. This is in somewhat similar to the Space Oblique Mercator (SOM) Projection developed for Landsat. However, Landsat has a very narrow swath, and the GTOC projection better preserves scale across a wider swath, making it more suited for sensors such as VIIRS. Also, unlike the SOM, it keeps the ground-track centered, which is a more efficient use of screen area when it is viewed. This paper describes details of the algorithm, including adjustments necessary for an elliptical orbit and an ellipsoidal Earth. It evaluates Map parameters including conformality and scale preservation, comparing this with other projections, including the SOM. It also evaluates improvements in efficiency relative to a search algorithm.